[mwessman]

I have W140 S500 ´98 and I have one problem. The car is jerking a bit while I`m driving at constant speed. It doesn´t matter whether the cruise is on or not. Idle is OK. Authorized workshop didn´t find any faulty codes. Do You have any ideas?

 

[hellah fresh]

tuneup?

 

[samosali]

fuel pump? is there any constant buzzing noise with contact on ?(do not start the car, just turn the ignition key to one step before starting). "healthy" buzzing for several seconds is ok, it means the pump(s) is ok.

fuel filter? replace it anyway ....

spark plugs? so cheap thing to replace it.

fuel pressure regulator (under the hood). i do not know anything about it, but its name tells everything.

rotors and distributor caps

vacuum and pressure leaks in the hoses under the hood

wires (engine wire harness, spark plug wires ...)

ETA (electronic throttle actuator), but this can be checked after all things above.

That is basic ... You must be aware that fault codes cannot be found out for many crucial parts of the car, for instance spark plugs.

 

[mwessman]

Fuel pump sounds normal
I should change the filter
I would like to hear more about the regulator
Spark plugs are new
The car doesn´t have distributor/rotor. It has eight ignition coils
I`ll ask more about ETA, thanks!

 

[samosali]

search here on this forum for "fuel regulator" or "fuel pressure" , etc. I am sure you will get ideas. sorry for the idea about rotors/caps, my car is 1992, and I forgot that technology improved over time

ETA opens a hatch which increases (decreases) amount of air into the engine. if this procedure is disturbed, then you have troubles. For instance if the hatch is hintered from any reason (maybe it is dirty and it cannot move normally) then the car will jerk. there is also a potentiometer inside the ETA (at least it is so in my car) which worns through time and causes the problems. the medicine for this is remanufacturing of ETA (I did it at BBA-reman from Hamburg, germany).

But this is only guessing. Unfortunately there is plenty of possibilities, and to check each of them you have to risk your time and money. That's why owning of an old car is not easy.

 

[SteveUrban]

You could have a leaky coil or two, shorting out.

Cheers,
Steve

 

[bayhas]

as mentioned above, perform a basic tune up, replace spark plugs with plugs from the dealer or authorized distributor, make sure you get resistor-less plugs. for some reason Autozone and advanced auto parts sell R sparkplugs for these cars.

Check for vacuum leaks at the intake manifold, dispite Samosali's opinion, fuel and fuel pressure issues are generally less common with these cars...

Replace your air filters.

 

[samosali]

as mentioned above, perform a basic tune up, replace spark plugs with plugs from the dealer or authorized distributor, make sure you get resistor-less plugs. for some reason Autozone and advanced auto parts sell R sparkplugs for these cars.

Check for vacuum leaks at the intake manifold, dispite Samosali's opinion, fuel and fuel pressure issues are generally less common with these cars...

Replace your air filters.

I agree: air filter is also important. And check the rubber (plastic) connector of MAF to ETA and then check the leaks between ETA and intake manifold like suggested by bayhas. There are 4 screws which fix ETA to this manifold (I think ca. 15 Nm torque) ... hard to believe but maybe they are loosen. First you will have to remove MAF from ETA, you can search here how to do it (not difficult but be ready for some "fun").

You wrote that spark plug are new but maybe one of them is bad. If the jerking existed before spark plugs replacement, then it is ok, most probably your new spark plugs are ok but I would follow bayhas's idea. The idea about damaged coils is also acceptable and possible. I do not have any bad experiences with fuel pressure thus it was only an assumption.

But I am sure that valves and gaskets are ok, otherwise you would have problem with idle. And if your car is 98, then your wire harnesses should be ok.

 

[bobs]

+1 on the vacuum leak. There's a hose on the right side valve cover that can degrade in places you can't readily see and cause a monster vacuum leak. Here's a post I did on the subject a while back:

http://www.benzworld.org/forums/w140-s-class/1373430-check-hose.html

 

[samosali]

+1 on the vacuum leak. There's a hose on the right side valve cover that can degrade in places you can't readily see and cause a monster vacuum leak. Here's a post I did on the subject a while back:

I am not sure if this hose can cause "real" vacuum leak which would affect the engine functioning. Now I am not 100% concentrated but if that is the hose which I have in mind then it is "only" for burning oil vapors from the engine ... so it serves for some kind of oil vapors overflow. Those oil vapors also contribute in smearing of pistons. So it is actually not meant to hold vacuum or pressure.

If I wrote something stupid someone will correct me. I just see an analogy between this hose and similar hoses in other cars. So, to conclude, I think there is no chance that this hose would cause car jerking ... which is actually a good news.

 

[lcmlee]

The same, little stall, while traveling at around 1800 rpm...I can feel the slight stall for a few seconds and then the car continues smoothly as usual. I guess I better start cross referencing all the work that has been done to my W140 and replace those parts that have yet to be changed out.